import datetime


def days(str1,str2):
    date1 = datetime.datetime.strptime(str1[0:10],"%Y-%m-%d")
    date2 = datetime.datetime.strptime(str2[0:10],"%Y-%m-%d")
    if date1 >=date2:

        return None
    num = (date2-date1).days

    return num